The present invention relates to power regulation circuits and, more particularly, to over-current protection circuits for power regulations circuits.
Power regulation circuits such as, for example, voltage regulators, typically include an over-current protection circuit. Conventional over-current protection circuits typically sense the output current and then disable or shut off the power regulation circuit. In some applications, the pass element is operated in the saturation and the linear or triode regions (in a power saving mode).
One over-current protection approach uses a sense resistor connected in series in the output current path. This approach includes circuitry to detect the current level through the series sense resistor and in response thereto to control the gate voltage of the pass device. This approach does not work well in applications in which the pass element operates in the triode or linear region and in large current applications and is sensitive to temperature.
In another approach, a transistor has its gate and drain connected to the source and gate, respectively, of the pass element. A series sense resistor is connected between the gate and source of the transistor. The current through the sense resistor thereby controls the conductivity of the transistor, which in turn helps control the gate voltage of the pass element when an over current condition exists. This approach also does not work well in large current applications and, in addition, is relatively sensitive to temperature, body effect and the value of the series sense resistor. This approach also does not work well in applications in which the pass element is operated in both the saturation and linear or triode regions.
In still another approach, a current mirror provides current to a smaller transistor that has its gate and source connected to the gate and source of pass device, respectively. The current mirror serves as a current to voltage converter that generates a voltage related to the output current. This voltage controls the gate of another transistor that in turn helps to control the gate voltage of the pass element when an over current condition exists. This approach is relatively sensitive to temperature, the Early effect, and the body effect. In addition, this approach tends to operate improperly when the pass element operates in the linear or triode region.
In view of the shortcomings described above, an over-current protection circuit is needed that works well in relatively large current applications with the pass element being operated in both the saturation and the linear or triode regions, and has reduced sensitivity to temperature, the Early Effect, the body effect, and other process parameters.
In accordance with aspects of the present invention, an over-current protection circuit for a power regulation circuit is provided. In one aspect of the present invention, the over-current protection circuit includes a reference generator that provides a reference level that is related to the level of the power source output that is controlled by the power regulation circuit in providing a regulated output. During normal operating conditions, the over-current protection circuit is isolated from the control signal provided to the pass element by other circuitry of the power regulation circuit. When the output current exceeds a predetermined level, the over-current protection circuit alters the control signal to the pass element to reduce the output current to a level below the predetermined value. If the over-current condition is removed, the over-current protection circuit detects this condition and again isolates itself from the control signal. The over-current protection circuit does not use a series sense resistor, which allows it to be used in relatively high current applications. In addition, because the reference generator tracks the level of the power source output, the over-current protection circuit can work properly when the pass element is operated in both the saturation, subthreshold and the linear or triode region.
In a further aspect of the present invention, the over-current protection circuit also includes a follower and a detector. The follower senses the level of the power regulation circuit output and, together with the reference generator, provides a clamp signal having a level equal to the level of the power regulation circuit output plus the reference level provided by the reference generator. Thus, the clamp signal depends on levels of both the power regulation circuit output and the power source output. The detector detects when the level of the control signal exceeds the clamp signal. If the level of the control signal does not exceed the level of the clamp signal, the detector does not significantly influence the level of the control signal. However, if the level of the control signal does exceed the level of the clamp signal, the detector operates to change the level of the control signal to reduce the output current.
In another aspect of the present invention, the follower senses the level of the power regulation circuit output and, together with the reference generator, provides a clamp signal having a level equal to the level of the power regulation circuit output plus the reference level provided by the reference generator. In this aspect, the detector detects when the level of the control signal drops below the clamp signal. If the level of the control signal exceeds the level of the clamp signal, the detector does not significantly influence the level of the control signal. However, if the level of the control signal drops below the level of the clamp signal, the detector operates to change the level of the control signal to reduce the output current.
In yet another aspect of the present invention, the reference generator is configurable or programmable to set the clamp signal to a desired level, thereby setting the maximum level of the output current. In one embodiment, the reference generator includes a smaller version of the pass element (i.e., a replica pass element), with a programmable constant current source biasing the replica pass element. The current source can be implemented with current mirror with parallel transistors that can be selectively enabled to control the amount of bias current.